The present invention relates to communication systems. More particularly, it concerns optical communications systems in which a plurality of terminal devices communicate with one another over a common optical pathway using frequency distinct channels.
In the communications system art, it is known to create networks in which a frequency spectrum is divided into unique channels of assigned bandwidth and center frequency and in which terminal devices are coupled into a network bus with each terminal device having a controllable local oscillator which can be tuned to acquire any one of the available channels to establish communications. The local oscillators can be of the type that are tuned in a continuous manner across the spectrum from one channel center frequency to another or, more preferably, of the type that respond to a "tuning event" to shift frequency from one channel center frequency to another. This type of system functions well where the frequency of the local oscillator source at each terminal device can be accurately controlled. In the radio frequency spectrum, inexpensive and stable oscillator sources which can be tuned over a wide frequency range, in either a continuously variable manner or in a step-wise channel-by-channel manner, are readily available. In the optical spectrum, however, stable oscillator sources which can be tuned over a wide range are generally not available. While inherently stable, fixed frequency devices are available in the form of gas lasers, such as the He--Ne laser, the use of gas lasers as frequency reference sources for the terminal devices is considered too expensive to allow implementation of networks on a large scale. While semiconductor lasers are available as local frequency sources in the optical spectrum, these devices are susceptible to both short and long term frequency variations as a function of temperature, drive current, and optical feedback and thus limit the ultimate channel density that can be achieved for a given spectrum.
Accordingly, a need exists for a communications system operating in the optical spectrum in which each terminal device in the system can acquire any available channel in a reliable and repeatable manner using inexpensive and stable frequency sources.